The present invention relates to an apparatus for forming a thin film which is suitable for forming a metal thin film or a semiconductor thin film which constitutes an IC, LSI or the like.
Various means for forming a thin film on a substrate have conventionally been proposed and various methods have been adopted therefor CVD and PVD, for example, are typical of those methods.
CVD is advantageous in the strong reactivity, while PVD is advantageous in that it enables film formation in a high vacuum, thereby forming a dense and strong film. On the other hand, CVD disadvantageously requires the temperature of the substrate to be kept high, and the formation of a reactive thin film is difficult by PVD except for a part of the ion plating methods, as is well known to those skilled in the art.
An ion plating method is a method of ionizing and evaporating in vacuum the substance evaporated in an active gas or an inert gas by producing a high-frequency electromagnetic field between a source of evaporation and substances being evaporated. The ion plating method includes DC ion plating method in which a DC voltage is applied between the source of evaporation and the substances being evaporated. These ion plating methods are disclosed in, for example, Japanese Patent Publication No. 52-29971 (1977) and 52-29091 (1977).
Japanese Patent Application Laid-Open (KOKAI) No. 59-89763 (1984) filed by Applicant discloses an apparatus for depositing a thin film. This apparatus is provided with a counter electrode for holding a substrate for deposition, and a grid disposed between the counter electrode and a source of evaporation which opposes the counter electrode in such a manner that the grid is impressed with a positive potential relative to the counter electrode. A filament for thermionic emission is disposed between the grid and the source of evaporation. This structure enables the substances evaporated from the source of evaporation to be ionized by the thermions emitted from the filament. The ionized substances which pass through the grid are accelerated by the effect of an electric field directed from the grid to the counter electrode and impinge on the substrate, thereby forming a thin film having a good adhesion on the substrate.
There are many other methods of and apparatuses for forming a thin film. However, the conventional methods of forming a thin film have some problems.
A first problem is that the adhesion between the thin film formed and the substrate is weak.
A second problem is that it is difficult to form a thin film on a substrate having a low heat resistance.
A third problem is that if a material having a low electric conductivity and having a large area such as a plastic film and a disk is used as a substrate for deposition, there is difference in physical properties of the thin film formed such as the thickness and the electrical resistance between the central portion and the peripheral portion of the substrate.
In a conventional apparatus for forming a thin film having the third problem, the reason why the thin film formed produces difference in physical properties such as the thickness and the resistance between the central portion and the peripheral portion of the substrate for deposition is as follows:
The surface of the counter electrode has an equal potential at all points and the electric field is orthogonal to the surface However, when the substrate for deposition which is thick to a certain extent and inferior in the electric conductivity is held by the counter electrode, the electric field on the surface of the substrate is not always orthogonal to the surface of the substrate, so that the states of the electric field are not the same between the central portion and the peripheral portion of the substrate.
The nature of the plasma generated at the central portion of the substrate is therefore different from that generated at the peripheral portion of the substrate, which fact leads to the difference in the physical properties of the thin film formed such as the thickness and the resistance between the central portion and the peripheral portion of the substrate for deposition. Especially, in the case of the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 59-89763 (1984), when the pressure of the gas introduced at the time of film formation is low or when the distance between the grid and the substrate is short, the evaporated particles reach the substrate while the dispersion of the particles due to the ambient gas or ions is insufficient, so that the film thickness is insufficient at the portion at which the material constituting the grid shields the evaporated particles.